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AN EXPERIMENTAL MEASURING
INSTRUMENT FOR INTERNAL STRESS
IN CONCRETE

Thesis for 1419 Degree of M. S.
MICHIGAN STATE COLLEGE

WiIIIam J. Mac Creery
1950

THES‘”

Date

0-169

 

i MICHIGAN STAT TEU ITY LIBRARI IES

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312

 

 

 

 

 

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This is to certify that the

thesis entitled
AN EXPERIMENTAL

MEASURING INSTRULENT FOR INTERNAL

STRESS IN CCNCRETE
presented by

NIL LIAM J. MACCREEHY

has been accepted towards fulfillment

of the requirements for

M. S. degree in CIVIL ENGINEERING

WM;

Major professor

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AN EXPERILENTAL LEASCRING INSTRUIENT
FOR INTERNAL STRESS IN CONCRETE

By

William J. MacCreery

A THESIS
Submitted to the School of Graduate Studies of Kichigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements

for the degree of

EASTER OF SCIENCE

Department of Civil Engineering

1950

THESIS

ACME;T C." LEDGEIEI‘J

The author wishes to thank Professor Cade, Professor
miller and Professor Blomquist of the Civil Engineering
Department for their assistance and advice on this thesis;
also the Soil Science Department for aiding in obtaining
the equipment used in the experimental part of this

thesis problem.

.1
{PS «I "3 9

TABLE OF CONTENTS

Introduction . . . . . . . . . . .
Theory . . . . . .
Procedure . . . . . .

Testing Equipment

EXperiment No. l . . . .
Experiment No. 2 . . . .
EXperiment No. 5 . . . . . .
Experiment No. 4 . . . . . .

Results. . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . .

Bibliography . . . . . . . . . . .

AN EXPERILENTAL LEASURING INSTRUKENT
FOR INTERNAL STRESS IN CCECRETE

Very little study has been made of the stresses in
concrete during the first ten hours after mixing or to the
time of final set. This thesis is an experiment designed
to get a measure of these internal stresses set up and the
rate at which they increase with the age of the concrete.

It is generally agreed that the internal stresses are
caused by the hydration of the cement and the volume
changes during this period. The major concern in volume
will not be the actual volume change but, rather, the causes
of the change. The hydration of cement is not fully under-
stood, chemically, but the author is concerned more with
the physical phenomenon and the part it plays in causing

internal stresses in the concrete.

THEORY

The first step in the measurement of internal stress
in concrete is to study the causes and understand the
action which produces the stress.

The best way to study the causes of internal stress is
to picture the formation of the internal pore structure of
the concrete from the time of mix to the final set. Im-
mediately after the concrete has been mixed and placed,
the solid particles, including the cement particles, are
in an unstable equilibrium and begin to settle. This settle-

ment causes the forcing of water upward to the surface,

developing a series of water channels up through the con-
crete. Gradually, the larger pieces of aggregate stabil-
ize by point contact or some other means and form a skeleton
inside which settlement is still taking place. The mortar
settlement forces more water upward, some of which lodges
under the aggregate, forming water pockets. Finally between
the sand grains, the cement settles out of the water-cement
mixture, leaving water voids above the cement paste. This
settlement, leaving water voids, makes the mixture non-
homogeneous and full of little inter-connecting water
passageways.

The next change comes from the hydration of the cement
or the forming of gel by the cement and water. The absolute
volume of the cement and water apart is greater than the
hydrated product, giving the first indication for cause of
internal stress. As this paste hardens it decreases in
volume and also uses up the_water of settlement, leaving
vacuum pockets which tend to put the paste in tension. Be-
fore the gel hardens sufficiently to resist these pockets,
it will have plastic movement but as the gel hardens it
resists the internal vacuum causing tension. The gel har-
dens around the aggregate causing it to be in compression
in order to resist the tension force in the hardened gel.
The air in voids of the aggregate is diffused through these
vacuum pockets, tending to help stabilize the mass somewhat
but has very little effect on the tension because of its

small amount. Some of the water is absorbed on the aggre-

gate surfaces, but this amount is considered to be very
small due to the total aggregate surface being so small in
comparison to its bulk.

As the concrete ages from the initial set when gel
formed, towards the final set, the bleeding or surface water
forced by settlement is used up by the hydration of the
cement and disappears completely. This hydration continues
inside the concrete as long as water is available up to the
point where all the cement has been used. This point has
been found to occur about 24 hours after initial set and
does not enter into this experiment.

According to the explained theory, an internal stress
should begin shortly after the initial set and increase to
the final set and beyond. In using this theory, the con-
cretes tested should be under the same conditions of tem-
perature, mixture, time of mix, and curing after testing.

Two types of concrete were tested: (1) Standard Port-
land cement and, (2 High Early Strength cement.. According
to this theory, it is believed that the High Early Strength
cement would show a faster rate of stress increase because
the cement grains are finer, allowing more and faster
water absorption, thus making a higher rate of hydration
take place within the concrete. Two consistancies of each
type were tested to see if the amount of water present
would have an effect on the rate of hydration, time of
starting, and the internal stress produced. The two con-

sistancies used for both types were (1) mix with 2-inch

- 5 -

slump and a water-cement ratio of .455 and (2) a mix with
an 8-inch slump and a water-cement ratio of .54. Based on

the preceding, the following procedure was developed and

used.

PROCEDURE

The first problem encountered was the development of
an instrument which could be placed in the concrete and,
in some way, give measurements of the stress developed.
Going back to the theory of the experiment, it is recalled
that the coarse aggregate was surrounded by mortar which
was in tension, thus producing a compressive force to
resist this tension. If this compression could be measured,
it would give a measure of the internal force caused by
the tension in the cement which is the cause of the inter-
nal stress.

Two ideas were born here - both using the compressive
force of the resisting aggregate as its basis for measure-
ment.

1. The idea was to embed in the concrete a small
cylinder of soft metal and get a measurement of the strain
which could readily be converted to stress. The cylinder
was 1-1/2 inches long, 1/4 inch in diameter, with stiff wire
protruding from each end out of the concrete for strain
measurement. The wire was protected by a thick rubber tube
which covered the end of the cylinder also, allowing pres-
sure only on the body surface. As the body was compressed,
the change in length was to be recorded.

2.. This idea was similar to the first, using a ball
of soft metal instead of a cylinder.

With some study and one experiment with the cylinder,
these two ideas were quickly discarded. The concrete was

- 5 -

too plastic until just about final set to give an appre-
ciable strain and, therefore, that type of instrument was
not usable for measurement between initial and final set.

The next idea, (the one used in this study) is based
on the fact that a vacuum is created by the hydration of
cement and water. If a measure of this pressure could be
made, it would give a measure of the internal forces inside
the concrete, eSpecially the tensile force in the hardened
gel. The problem was to get a measure of this negative
pressure. This problem was solved by an apparatus used in
the Soil Science Department, called a tentiometer. The
apparatus consists of: (l) a manometer with mercury, (2) a
measuring device (centimeter stick), (5) plastic tubing, and
(4) porcelain cup which will emit water through to a sur-
rounding medium. These were purchased from a special com-
pany in New Jersey.
TestinggEquipment

The manometer was set up on a standard and a piece of
plastic tubing was fitted on the and Opposite the mercury
cup. The fitting was Y-shaped so that water could be in-
jected into the system through one limb while the other two
ran from the manometer to the plastic tubing. The porcelain
cup was attached to the other end of the tube and placed in
a pail of sand. Boiled distilled water was injected into
the whole system, making sure that there was no air entrapped.
Lercury was put into the cup up to the "0" point on the

meter stick. The whole system was made air-proof by covering

_ 5 -

all joints with a thick coating of rubber cement. The cup
had been tested previously for porosity by applying an air
pressure of 25 pounds to it while in water. The air just
barely came through, thus insuring it against too much
porosity. This was found by the Soil Science Department
to be the best test. The cup was then placed in a container
of distilled water to soak for 24 hours before use in the
test.

The test run with sand proved consistant with previous
tests so that the equipment and experiment set-up was con-

sidered ready.

Experiment No. l - Standard Portland Cement - 2“

The first eXperiment was run on a Standard Portland
ceaent with a 2-inch slump. Two standard test cylinders

of 6-inch diameter were made, using the following quan-

tities:
Water 4.1 lb.
Cement 9.0 lb.
Fine Aggregate 20 lb.
Coarse Aggregate 59 lb.
W/C .455
Slump 2 inches

The test equipment was set except for filling with
water when the concrete was made so there was no delay.
One cylinder was set near enough to the equipment so that
the porcelain cup attached to the plastic tubing could be
placed in the fresh concrete. The cup was placed approx-
imately in the center of the test cylinder and the concrete
surface smoothed. The system was then filled with the
boiled distilled water and the experiment started. Boiled
distilled water was used in order to make sure that there
was no free oxygen in the water and also to get rid of all
impurities which might hamper the passage of water through
the porcelain cup.

An initial reading was taken and another reading one
hour later; then readings every 5 minutes for the next 4
hours. From then on, readings were taken every 10 minutes
up to 15 hours. Another reading was taken at 22 hours but

- 8 -

vas considered void for reasons which will be eXplained
later.

The second cylinder was placed beside the first and
then, after the test, both were placed in the moist room
for curing and a 14-day test. This procedure insured
both cylinders being cured alike.

A curve was plotted with the ordinate in centimeters
of mercury and absissa in hours. After plotting the
curve, it was found that readings taken at 20-minute inter-
vals would be sufficient for plotting accurate curves.

The initial set occurred at approximately one and
one-quarter hours after mixing and the final set came
about ten hours after mixing. The curve, during these
hours, definitely shows a presence of stress and also gives

its rate of increase and when it occurred.

DATA
EXP. KO. 1 - STANDARD PORTLAND CEXEIT - 2-INCH SLEEP

Time (hr:min) Reading (cm.hg) Corr. Reading Interval

 

Initial
1.500 2.20 -1050 005
2:00 5.45 1.75 5.25
2:45 6.80 5.10 1.55
5:00 7.65 5.95 .85
5:10 8.10 4.40 .45
5:20 8.70 5.00 .60
5:50 9.10 5.40 .40
5:50 10.50 6.80 .70
4:00 11.00 7.50 .50
4:20 12.50 8.80 1.50
4:40 14.50 10.60 1.80
5:00 16.50 12.80 2.20
5:20 19.00 15.50 2.50
5:40 21.60 17.90 2.60
6:00 24.55 20.65 2.75
6:20 27.15 25.45 2.80
6:40 29.50 25.80 2.55
7:00 51.50 27.80 2.00
7:20 55.40 29.70 2.90
7:40 55.65 51.95 2.25
8:00 59.50 55.60 5.60
8:20 42.65 58.95 5.55
8:40 45.95 42.25 5.55
9:00 48.90 45.20 2.95
9:20 51.60 47.90 2.70
9:40 55.95 50.25 2.55
10:00 55.80 52.10 1.85
10:20 57.60 55.90 1.80
10:40 59.15 55.45 1.55
11:00 60.55 56.85 1.40
11:20‘ 61.70 58.00 1.15
11:40 62.90 59.20 1.20
2:00 65.50 59.80 .60
24:00 69.50 65.60

 

-10..

Correction - 50 cm. water 3 5.7 cm..Hg.

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Experiment No. 2 - Standard Portland Cement - 8"

The second experiment was run on Standard Portland
cement with an S-inch slump. This was chosen to see if
the amount of water present would affect the stress.

A new porcelain cup was prepared and soaked 24 hours
in advance and the test apparatus set up. Two test cylin-

ders were prepared using the following quantities:

Water 4.9 lb.
Cement 9.0 lb.
Fine Aggregate 21 lb.
Coarse Aggregate 56 lb.
W/C .54

Slump 8 inches

The equipment was readied and the cup buried half way
down in one of the test cylinders. Readings were taken
every 20 minutes up to about 4 hours after mixing. Then
a leak developed at one of the joints of the "Y" and the
test was no good. The cup was dug out of the concrete,
cleaned, tested, and set to soak again. Another batch of
concrete was mixed the following day, conforming to the
above specifications and the test started again. his
time, it was successful and a curve was obtained which was
very similar to that in eXperiment No. l. The curves had
to be corrected because the cup level was below the mercury
“0“ mark approximately 52 centimeters, giving a head on

the cup or positive pressure of 2.4 cm. of mercury.

- 12 -

DATA
EXP. N0. 2 - STANDARD PORTLAND CEKENT - 8-INCH SLUMP

 

Time (hr:min) Reading (cm.hg) Corr. Reading Interval
Initial

0:00 1.00 -l.40 reading,
:20 1.40 -1.00 0.40
:40 2.10 ~0.50 .70
1:00 2.90 0.50 .80
1:20 5.70 1.50 .80
1:40 4.50 2.10 .80
2:00 5.50 2.90 .80
2:20 6.10 5.70 .80
2:40 7.00 4.60 .90
5:00 8.10 5.70 1.10
5:20 9.60 7.20 1.50
5:40 11.20 8.80 1.60
4:00 5.10 10.70 1.90
4:20 15.15 12.75 2.05
4:40 17.80 15.40 2.65
5:00 20.70 18.50 2.90
5:20 24.10 21.70 5.40
5:40 27.80 25.40 5.70
6:00 50.60 28.20 2.80
6:20 55.50 51.10 2.90
6:40 56.50 55.90 2.80
7:00 41.70 59.50 2.60
7:20 44.50 41.90 2.60
7:40 47.20 44.80 2.90
8:00 49.50 46.90 2.10
8:20 51.60 49.20 2.50
8:40 55.70 51.50 2.10
9:00 55.50 55.10 1.80
9:20 57.10 54.70 1.60
9:40 58.80 56.40 1.70
10:00 60.10 57.60 1.20
10:20 61.40 58.90 1.50
62.50 60.00 1.10
65.40 60.90 0-90

68.50 66.10

 

-13...

Correction = 54 cm. H20 or = 2-4 cm. H8.

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Experiment No. 5 - High Early Strength - 2-1/2“

The third eXperiment was run on High Early Strength
cement with a 2-1/2 inch slump. A porcelain cup was pre-
pared and soaked for 24 hours. Then the equipment was

set up and the mix prepared with the following quantities:

hater 4.1 1b..
Cement 9.0 lb.
Fine Aggregate 20 1b.
Coarse Aggregate 59 lb.

W/C .455

Slump 2% inches

wo test cylinders were filled and tamped. The cup
was buried in one and the experiment started. Two fail-
ures occurred due to air leaking in but the third attempt
proved successful and a curve was obtained. The porcelain
cup was usable after both failures because they occurred
2 hours and 5 hours reSpectively after mixing and the con-
crete was not sufficiently hardened to prevent recovery.
The cup was tested and soaked each time to make certain
that the next test would be valid. After the experiment,
the cylinders were removed to the moist room for curing

and the 7-day test.

1232.4
EXP. Ho. 3 - HIGH EARLY STRENGTH CELENT - ayelrmw sum}

Time (hr:min) Reading (cm.hg) Corr..Reading Interval

 

-16...

Initial

0:00 0.50 -1.90 reading.
:20 0070 “1070 0020
:40 0.90 -1.50 .20
1:00 1030 -1.10 040
1:20 2:00 -0.40 .70
1:40 2.60 0.20 .60
2:00 2.85 0.45 .25
2:20 5.40 1.00 .55
2:40 5.70 1.50 .50
5:00 5.20 2.80 1.50
5:20 6.80 4.40 1.60
5:40 9.00 6.60 2.20
4:00 11.70 9.50 2.70
4:20 15.80 15.40 4.10
4:40 20.90 18.50 5.10
5:00 26.50 25.90 5.40
5:20 51.90 29.50 5.60
5:40 57.70 55.50 5.80
6:00 45.80 41.40 6.10
6:20 50.00 47.60 6.20
6:40 55.80 55.40 5.80
7:00 60.00 57.60 4.20
7:20 65.10 60.70 5.10
7:40 65.40 65.00 2.50
8:00 66.50 64.10 1.10
8:20 67.20 64.80 0.70
8:40 67.75 65.55 .55
9:00 68.05 65.65 .50
9:20 68.25 65.85 .20
9:40 68.55 65.95 .10
10:00 68.45 66.05 .10

Correction 3 cm. mercury

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ggperiment No. 4 - High EarleStrength - 8-1/2"

.\

High Early Strength concrete with an 8-1/2 inch slump
was used for the fourth experiment. The same procedure was

followed as before, using the following quantities:

Water 5. lb.
Cement 9.1 lb.
Fine Aggregate 21 lb.
Coarse Aggregate 56 lb.

W/C .55

Slump 8% inches

The first trial was successful and, to insure against
any air getting into the system through the concrete, the
cylinder was sealed in paraffin. Readings were taken every
20 minutes up to final set and the curve plotted. The cyl-
inders were put in the moist room and the equipment dis-
assembled.

Following is a diagram of the eXperimental equipment

and how it was used.

- lg -

DATA
EXP. N0. 4 - HIGH EARLY SQREHGTH CEKEKT - 8-1/2-INCH SLUEP

Time (hrzmin) Reading (cm.ig) Corr..Reading Interval

 

Initial
0:00 1.00 -1.4 reading
:20 1.10 -1.3 0.70
:40 1050 -lol 02
1:00 1.60 “008 030
1:20 1.90 -0.5 .30
1:40 2.10 “003 020
2:00 2.30 -0.1 .20
2:20 2.40 0 .70
2:40 2.50 0.10 .20
3:00 2.80 0.40 .30
3:20 3.60 1.20 .80
3:40 4.80 2.40 1.20
4:00 6.3 3.90 1.50
4:20 7.60 5.20 1.30
4:40 9.40 7.00 1.80
5:00 11.60 9.20 2.20
5:20 14.60 12.20 3.00
5:40 17.90 15.50 3.30
6:00 21.40 19.00 3.50
6:20 24.90 22.50 3.50
6:40 28.70 26.30 3.80
7:00 32.40 30.00 3.70
7:20 35.80 33.40 3.40
7:40 39.50 37.10 3.70
8:00 43.30 40.90 3.80
8:40 50.60 48.20 3.70
9:00 53.90 51.50 3.30
9:20 57.20 54.80 3.30
9:40 60.40 58.00 3.20
10:00 62.90 60.50 2.50
10:20 64.70 62.30 1.80
10:40 66.00 63.60 1.30
11:00 66.80 64.40 .80

 

Correction = 2.4 cm. mercury

-19..

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RESULTS

The testing equipment was found to function prOperly
up to the time of final set of the concrete, after which
air seeped into the system from Within the concrete, making
the readings erroneous. The negative pressure pulling the
water into the concrete was great so that the system even-
tually tried to equalize this negative pressure by replacing
the used water with air diffusing back through the porce-
lain. This phenomenon uas very evident by air bubbles
appearing in the plastic tubing coming from the cup buried
in the concrete. By 24 hours after the beginning of the
test, the plastic tube was completely filled with air and
a final reading of approximately 62 centimeters of mercury
was observed, as shown on the graphs.

This phenomenon caused the curve to slope off, also
making the upper part of the curve useless. A line tangent
to the largest slope was drawn and extended on up, giving
the greatest increase in stress per unit of time occurring
within the specified concrete between the initial and
final set.

EXperiment No. l

 

The Standard Portland cement with a 2-inch slump gave
the following results:
Kax. stress at final set 50 cm. or 9.67 psi.

Kax. stress increase (slope) 8.5 cm/hr. or 1.64 psi/hr.

Experiment No. 2

Standard Portland cement with an 8-inch slump gave a
curve with a hump in the middle, so two tangent lines were
drawn. Actually, the second (the one of least slope) is
more likely to be the most accurate one due to more water
which, theoretically, slows up stress development. The
results are as follows:

Max. stress at final set 48 cm. or 9.5 psi.
(a) Hex. stress increase (slope) 10.5 cm/hr. or 2.05 psi/hr..
(b) Kax. stress increase (slope) 7.0 cm/hr. or 1.55 psi/hr.
Experiment No..5

High Early Strength cement with a slump of 2-1/2 inches
showed a very marked increase in the rate of stress develop-
ment. Actually, it was eXpected because of the shorter
time of final set and high early strength developed. The
results are as follows:

Kax. stress at final set 55.5 cm. or 10.75 psi.

Kax. stress increase 18.5 cm/hr.or 5.58 psi/hr..
Experiment No. 4

High Early Strength cement with a slump of 8-1/2 inches
showed, definitely, the retarding effect of an excess of
water on the ate of stress development and the time of
final set. The results are as follows:

Max. stress at final set 55.5 cm. or 10.55 psi..

Eax. stress increase 11.0 cm/hro or 2.15 psi/hr.

The concrete mixes used in this experiment were taken

from a set of mixes which had been previously tested and had

-25..

strength curves for each mix. This was done so that the
test cylinders on this experiment could be tested after
14 days curing and the results correlated with the 28-day
strength, thus giving the strength of the concrete.

The strength of concrete, as found by the 14-day test

and using the original curves of the mixes, were as follows:

.Elflé Slpmp Streggth
Standard Portland Cement 2-inch 5,800 psi.
High Early Strength Cement 2% inch 4,000 psi.
Standard Portland Cement 8-inch 2,900 psi.
High Early Strength Cement 8% inch 5,000 psi.

- 26 _

CONCLUSION

The results of this thesis problem have proved,
definitely, the existence of an internal stress developing
from the time of initial set. The measure of this internal
stress was successful up to the point of final set, which
covers the problem set up in this thesis. The rate at
which this stress develOped was established for the types
of cnncrete tested, also the retarding effect of a higher
water content on the stress development inside the specimens.

The internal stress was shown to develop quite rapidly
after the first few hours and indicates a definite reason
for cracks forming before final set of the concrete. The
rate of development was eSpecially high in the High Early
Strength cement as was expected, bearing out the theory
used in this thesis.

A retarding effect or slowing up of the rate of devel-
"opment of this stress was noted by the use of a higher
water-cement ratio in the mix. This effect was eSpecially
evident in the High Early Strength cement where not only
the rate of development of stress but also the time of
final set was retarded. This higher water-cement ratio did
not, however, have much effect on the stresses at the time
of final set, which are all between 45 and 55 centimeters
of mercury. The first four graphs were plotted in centi-
meters of mercury, while the fifth, a comparison graph,

was plotted in pounds-per-square inch and hours.

-27,-

The type of equipment used in this problem proved
sufficient up to the time of final set, after which it
gave erroneous readings due to air leaking in. It is
believed that for experimentation from final set and beyond,
the principle of the first two ideas expressed in the pro-
cedure might give results.

This thesis has covered a problem of experimentation
and research which, it is hoped, will help in the steps
of understanding and improving the disadvantages of stress
development which cause cracking and small failures during

the initial stage of concrete aging-

~28 -

BIBLIOGRAPHY

 

Carlson, R.W. "Chemistry and Physics of Concrete Shri kage"
ASTM Proceedings, Vol. xxxv, p. 560, 1955.. -

Rickett, G., "Shrinkage Stresses in Concrete“, AC1 Bul-
letin II, march, 1946.. -

Swayze, E. A., "Early Volume Changes and Their Control“,
ACI Proceedings, pg. 425, April, 1942. -

Concrete Manual, U» S. Dep't. of the Interior,
Bureau of Reclamation, 4th Edition, 1942.

Kurphy, "Properties of Engineering Katerials", 2nd Edition,
1948.. i

- 29—-

IN, 1;

HICHIGRN STQTE UNIV. LIBRQRIES

III II III"

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